I finally cared enough to dig in to why only a little bit of my map was showing up in OS X Preview or in Ghostscript.

Turns out that although the Bounding Box is correct, the display application is clipping to the default page size. For Ghostscript you can use the -dDEVICEWIDTHPOINTS and -dDEVICEHEIGHTPOINTS options (for my map, that's a forest-shredding 7200 and 2000 points, respectively). I suspect in Preview you can define a custom paper size and set that to the system default and then double-clicking the map will work fine (woe unto you if you actually try to print something, though, before changing it back). I'm not near an OS X machine right now to test, though.

Illustrator and Evince don't seem to care and are happy to show you the whole map regardless of your paper size. Evince is pretty standard on Linux, there appears to be a Windows build, and there's a Darwinports port of it, so it would be a reasonable free option on all platforms.

So, to summarize: I think it's worth mentioning that Preview.app and Ghostscript will by default clip the map at the default paper size, and that this can present a problem for large maps. However, the map is correctly generated, and the problem can be worked around by using Evince or by defining a suitably large page size.